This invention relates to a method of and apparatus for refrigerating a permanent gas. It is particularly but not exclusively concerned with cooling a relatively high pressure stream of a permanent gas to its critical temperature or below by heat exchange with relatively low pressure working fluid and is particularly applicable to the liquefaction of permanent gases.
A permanent gas has the property of not being able to be liquefied solely by increasing the pressure of the gas. Cooling of the gas at pressure is necessary so as to reach a temperature at which the gas can exist in equilibrium with its liquid state.
Conventional processes for liquefying a permanent gas or cooling it to below the critical point typically require the gas to be compressed (unless it is already available at a suitably elevated pressure, generally a pressure above the critical pressure) and heat exchanged in one or more heat exchangers against a relatively low pressure stream of working fluid. At least part of such stream of working fluid may be formed by compressing the working fluid, cooling it, typically in the aforesaid heat exchanger or exchangers, and then expanding it with the performance of external work (`work expansion`). The working fluid may itself be taken from the high pressure stream of permanent gas or the permanent gas may be kept separate from the working fluid. In the latter example, the working fluid may have the same composition as the permanent gas, or may have a different composition therefrom.
A graph of enthalpy per standard cubic meter of gas plotted against temperature for a permanent gas (herein after called an enthalpy-temperature or temperature-enthalpy curve) is shown in FIG. 1 of the accompanying drawings. Merely by way of example, the gas selected is nitrogen at a pressure of 50 atmospheres. The enthalpy-temperature curve runs from point A to point E. Point A is, say, at a temperature at which refrigeration of the gas may commence. Point E is at the temperature at which the gas has become an undercooled liquid. Starting at Point A and descending the curve, its first section is section A-B in which the gas approximates in behaviour to an ideal gas. Then there is a section B-C. In this section the behaviour of the gas deviates from that of an ideal gas and begins to assume some of the properties of a liquid. We call this section B-C the gaseous transitional section. The final section is section C-D-E. In this section the transformation from the gaseous to the liquid phase takes place and is completed.
As will be appreciated below, the section B-C of the curve is of key importance to our invention. The point B occurs where the rate of change in the slope of the curve becomes more pronounced. The slope of the curve at any temperature is the heat capacity (at constant pressure) of the gas per standard cubic meter at that temperature. We define point B as the point where the rate of change in the value of the heat capacity (at constant pressure) of the gas per standard cubic meter increases by about 1% per Kelvin as the gas is cooled. The point B defines the upper temperature limit of the gaseous transitional section.
The point C defines the lower temperature limit of the gaseous transitional section. Point C is at the temperature at which the rate of change with temperature of the heat capacity (at constant pressure) of the gas per standard cubic meter is at a maximum. If the gas to be refrigerated is at a pressure below the critical pressure the point C lies at the saturation temperature of the liquefied gas and is the point at which the gas begins to liquefy as it is cooled. For gases at pressures above the critical pressure, point C is by definition at a higher temperature than the critical temperature.
In FIG. 2 of the accompanying drawings, we identify the points B and C on a number of enthalpy-temperature curves for nitrogen at different pressures above and below the critical pressure.
In practice, at any given enthalpy value, there is a given temperature of the gas being cooled dependent solely on pressure. At each point a lower temperature is necessary in the working fluid. This temperature can be plotted on the temperature-enthalpy graph. It has been considered desirable to try to match the two temperature-enthalpy curves as closely as possible so as to minimize the area defined between the two curves. For example, in U.S. patent specification No. 3,358,460 the discrepancy between the two curves is identified as leading to the consumption of substantial amounts of power, making the refrigeration system inefficient. There is thus a disclosure of approximating the shape of the refrigerant curve to that of the permanent gas curve by causing components of the refrigerant stream to undergo a plurality of work expansion stages with intervening reheating. There is no substantive discussion in the U.S. patent specification of the theory of where best to deploy the work-expanded refrigerant. However, if FIGS. 2 and 3 of U.S. patent specification No. 3,358,460 are compared with one another, it can be seen that the bulk of the area between the cooling and heating curves of FIG. 2 comes well below the point where there is a maximum rate of change in the heat capacity (at constant pressure) per standard cubic meter (see our Figure for where this point lies) and accordingly both the work-expanded refrigerant streams are shown in FIG. 3 of the U.S. patent as being brought into heat exchange relationship with the stream being cooled at temperatures of the stream being cooled well below this point.